The field to which the present invention relates is that generally known as electrical discharge machining, sometimes hereinafter referred to as EDM, in which material is removed from an electrically conductive workpiece by the action of electrical gap discharges occurring between a tool electrode and a workpiece. A dielectric coolant fluid is circulated and recirculated through the gap, usually under pressure, throughout the machining operation. An electrode or a workpiece servo feed system is used to provide relative movement and thus maintain an optimum gap spacing between the electrode and the workpiece as the workpiece material is being removed.
It is important to the process of EDM that the machining power pulses provided at the gap are of closely and precisely controllable on-off time and frequency to insure repeatability of results and to provide appropriate cutting action for the type of operation being carried on. Various types of pulse generators which have this capability have been developed and are in commercial use for EDM. One commonly used type of EDM power supply includes as a principal part of its machining power pulse generator an astable multivibrator in which on-off time and frequency are controlled and preset by a ganged capacitor and resistor arrangement. One example of this type of pulse generator and an associated protection system is shown and described in Kurt H. Sennowitz, U.S. Pat. No. 3,649,802, issued on Mar. 14, 1972 for "Protective System for Electrical Discharge Machining Power Supply Circuit", which patent is of common ownership herewith.
A further arrangement for digital type EDM pulse generator is shown and described in Oliver A. Bell, Jr., U.S. Pat. No. 3,809,847, issued on May 7, 1974, for "Method and Apparatus for Electrical Discharge Machining".
A still further type of digital multivibrator is shown and described in Oliver A. Bell, Jr., U.S. Pat. No. 4,071,729, issued on Jan. 31, 1978, for "Adaptive Control System and Method for Electrical Discharge Machining". This patent shows an on and off time generator which receives inputs from a programmable computer and from this general arrangement provides machining power pulses to the machining gap. All the above noted patents are of common ownership herewith.
The present invention is particularly designed for use with a digital type pulse generator that is controlled by a programmable computer or similar input device. Reference is made to our copending U.S. Patent Application Ser. No. 068,328 filed on Aug. 21, 1979 for "Programmable Pulse Generator for Electrical Discharge Machining Apparatus".
The prior art has shown a number of current limiting systems for EDM in which a current limit is relay set in accordance with the frequency at which the pulse generator is operating. The present invention goes further and allows on and off times and current limit to be entered. The off time is then readjusted to an allowable maximum value for the current limit desired.